Method for fabricating fiber bragg grating elements and planar light circuits made thereof

ABSTRACT

A method for fabricating Fiber Bragg Grating elements and planar light circuits made thereof. A mask having a predetermined pattern and a wafer are provided, wherein a light-guiding channel filled with light-guiding substance is formed on the wafer. A photoresist layer is then formed to cover the wafer. Magnification of a photolithography apparatus is adjusted to a first magnification, followed by transferring the pattern on the mask to the photoresist layer to form a first pattern. Light-guiding substance not covered by the photoresist layer is then removed so that the first pattern is transferred to the light-guiding channel. The light-guiding channel then forms a Fiber Bragg Grating element.

This nonprovisional application claims priority under 35 U.S.C. §119(a)on Patent Application No(s). 092109982 filed in TAIWAN, R.O.C. on Apr.29, 2003, which is(are) herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for fabricating opticalelements, and in particular to a method for fabricating Fiber BraggGrating elements and planar light circuits made thereof.

2. Description of the Related Art

In long distance fiber optic communication systems, inactive elements,such as Fiber Bragg Grating (FBG), array waveguide grating (AWG) arecritical high-end elements. FBG applications, for example, includeoptical add-drop multiplexers (OADM), Erbium doped fiber amplifiers(EDFA) and Raman amplifiers, all of which have recently become verypopular.

Fiber Bragg Grating (FBG) is commonly manufactured by subjecting opticalfiber to high energy UV excimer laser. When parts of the optical fiberare subject to a high energy laser treatment, the bonding state of theinner molecular structure changes, thereby increasing the refractiveindex. The refractive index of the optical fiber is changed by forminggrating periods using masks. When a bandwidth is an integer times alight of specific wavelength, the light will be reflected and the restof the light passes through the optical fiber. As a result, the incominglight either passes through or is reflected by the Fiber Bragg Grating.In terms of input and output, the FBG is an optical notch filter,corresponding to a specific bandwidth (BW_(n)), a notch frequency(f_(n)) and a notch wavelength (λ_(n)).

The fabrication of Fiber Bragg Grating (FBG) elements is carried out bycontrolling the laser energy and laser exposure time, and employingmasks. FIG. 1 illustrates a traditional method for fabricating a FiberBragg Grating element. In FIG. 1, 10 represents optical fiber, 12 is amask, 14 is a reflective mirror, and 16 is a KrF excimer laser beam witha 248 nm wavelength. The KrF laser beam penetrates the mask 12, passesthrough the reflective mirror 14, and hits the optical fiber 10. The isinner molecular structure of the optical fiber 10 is thus changed toform interference stripes 20 of a specific reflective index. The gratingperiod (pitch) 22 controls the reflective index when an incoming lightpasses through the optical fiber. Therefore, when light 8 passes throughthe optical fiber 10, 24 a denotes the light corresponding to theinterference stripes is selected as the reflective light 24. Theremaining light, denoted as 26 in the figure, is then transmittedthrough the optical fiber.

Due to the restrictions of the current semiconductor manufacturingprocess, integration of Fiber Bragg Grating elements with semiconductorwafers does not always satisfy various design requirements. The reasonsfor this are further discussed in the following.

Current patterns on masks are defined by electron beams with a minimumwidth of e-beam of 50 angstroms. As a result, the minimum pitch ofpatterns on masks is 50 angstroms. When patterns are transferred ontowafers by photolithography apparatus, with a magnification of 5 forexample, the minimum pitch (i.e. resolution) on a wafer is 10 angstroms.In other words, if a Fiber Bragg Grating 2000 angstroms in wavelength isto be fabricated, using the masks and magnification described above, thereal wavelength must be 2000+n*10 angstroms. Consequently, an FBG of2005 angstroms in wavelength cannot be fabricated by this method.Therefore, design freedom is restricted.

Hence, there is a need for a novel method for fabricating FBG withoutlimitations on resolutions so that various design requirements can beobtained.

SUMMARY OF THE INVENTION

Accordingly, an object of the invention is to provide a method forfabricating Fiber Bragg Grating elements using photolithographytechniques commonly used in the semiconductor industry. The correlationof pattern pitch, and the magnification of steppers are fine tuned sothat patterns having predetermined grating periods are transferred to awafer to form interference stripes that select specific wavelengths. Thewafer having specific interference stripes then forms the Fiber BraggGrating element.

The method for fabricating Fiber Bragg Grating elements, comprises thesteps of: (a) providing a mask having a predetermined pattern and awafer, wherein a light-guiding channel filled with light-guidingsubstance is formed on the wafer, and a photoresist layer is formed onthe wafer; (b) adjusting the magnification of a photolithographyapparatus to a first magnification and transferring the pattern of themask onto the photoresist layer to form a first pattern; and (c)removing the light-guiding substance not covered by the photoresistlayer so that the first pattern is transferred to the light-guidingchannel, thus the light-guiding channel forms a Fiber Bragg Gratingelement.

According to the method of the invention, the mask preferably contains aglass substrate, and the predetermined pattern on the mask is preferablymade of Cr.

The method of the invention further comprises adjusting themagnification of the photolithography apparatus to a secondmagnification so that the predetermined pattern is transferred to thephotoresist layer to form a second pattern, wherein the secondmagnification is not equal to the first magnification, and the firstpattern and the second pattern are formed on the light-guiding channelwithout overlapping one another; wherein the first pattern and thesecond pattern are simultaneously transferred in step (c) to thelight-guiding channel.

The method of the invention is also useful in fabricating a planar lightcircuit (PLC) on a wafer. The PLC comprises: a light-guiding channel,formed on the surface of the wafer; and a plurality of Fiber BraggGrating elements formed in series on the light-guiding channel, and theFiber Bragg Grating elements contain corresponding patterns similar toeach other, but have different sizes.

In the planar light circuit mentioned above, the Fiber Bragg Gratingoptical elements correspond to a number of light wavelengths, whereinadjacent wavelength difference is less than 10 nm, or is less than abandwidth of the Fiber Bragg Grating elements.

Moreover, the plurality Fiber Bragg Grating elements are combined as anequivalent Fiber Bragg Grating element, which comprises an equivalentnotch wavelength and an equivalent bandwidth, wherein the equivalentbandwidth is greater than any bandwidth of the Fiber Bragg Gratingelements.

According to the method for fabricating Fiber Bragg Grating opticalelements of the invention, conventional problems of fabricating FBG ofinterpolation sizes is overcome by photolithography technique commonlyused in the semiconductor industry. Predetermined patterns aretransferred to wafers by finding suitable magnifications to meet designrequirements. By doing so, predetermined patterns on masks can betransferred to wafers by any magnification. Not only is high accuracyachieved, but non-integer magnification is also achieved.

According to the planar light circuit of the invention, variation duringthe photolithography process is prevented by forming a plurality ofFiber Bragg Grating optical elements in series. When a single FiberBragg Grating optical element cannot accurately select a specificwavelength due to variation during the fabrication process, it can beovercome by effectively increasing the bandwidth of the Fiber BraggGrating optical element to meet design requirements. In other words, theplanar light circuit, formed by a plurality of Fiber Bragg Gratingelements in series, exhibits enlarged bandwidth to accommodate potentialvariation during the fabrication process.

A detailed description is given in the following embodiments withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more fully understood by reading thesubsequent detailed description and examples with references made to theaccompanying drawings, wherein:

FIG. 1 is a cross section of a conventional method of fabricating FiberBragg Grating elements;

FIGS. 2A and 2B are schematic views and cross sections showing thefabrication of the first embodiment;

FIG. 3 illustrates the position of the Fiber Bragg Grating elementrelative to the wafer according to the first embodiment of theinvention;

FIGS. 4A and 4B illustrate top views and cross sections of thefabrication process according to the first embodiment of the invention;

FIGS. 5A, 5B and 5C are top views and cross sections of the fabricationprocess according to the first embodiment of the invention;

FIGS. 6A, 6B and 6C are top views and cross sections of the fabricationprocess according to the first embodiment of the invention;

FIG. 7 is a flowchart according to the first embodiment of theinvention;

FIG. 8A is a schematic view of the planar light circuit fabricated inthe second embodiment of the invention;

FIG. 8B is a graph showing the relation of the gain vs. wavelengthaccording to a conventional Fiber Bragg Grating element;

FIG. 8C is a schematic view of the planar light circuit fabricated inthe second embodiment of the invention;

FIG. 9 is a flowchart according to the second embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments

1^(st) Embodiment

FIG. 7 illustrates the flowchart according to the first embodiment ofthe invention.

First, in step S10A, a mask 120, as shown in FIGS. 2A and 2B isprovided. A pattern 122 formed with a predetermined pitch A is formed onthe mask. The mask 120 is preferably formed on a glass substrate. Thepattern 122 is preferably formed by Tin.

Meanwhile, in step S10 B, a wafer 100, shown in FIG. 3 is provided,wherein an area R indicates the position where the Fiber Bragg Gratingelement is to be formed. The area R comprises a light-guiding channel105, filled with light-guiding substance 110. In order to more clearlyillustrate the area R, FIGS. 4A and 4B illustrate the top view and crosssections of the area R. FIG. 4B is the cross section of FIG. 4A alongthe line Y–Y′. For example, wafer 100 is silicon. The light-guidingchannel 105 is a trench formed by SiN, wherein the light-guidingsubstance 110 is SiO.

Next, photoresist is coated onto the area R to form a photoresist layer140 by a coating machine in step S20.

Then, the magnification of a photolithography apparatus is adjusted to apredetermined first magnification in step S30, which is the criticalstep of the invention where magnification is adjusted to meet variousdesign requirements. In conventional methods, magnification is usually 4or 5, which means the patterns on the mask, transferred to the wafer,are reduced to ¼ or ⅕. In other words, magnification of thephotolithography apparatus of semiconductor process is fixed, and cannotbe changed when adopting conventional methods. As a result, photoresistlayers on wafers are continuously exposed and developed one by onewithout changing magnifications due to stability concerns in theconventional semiconductor manufacturing process. In order to solve thisfixed magnification problem, the invention features active modificationof the magnification of the photolithography apparatus, in order to meetvarious design requirements. Consequently, magnification can either be 5(integer) or with decimals, such as 5.02.

Next, in step S40, exposure/development using the predeterminedmagnification is carried out to transfer the pattern 122 to thephotoresist layer 140 to form a first pattern, as shown in FIGS. 5A˜5C.FIG. 5A illustrates the top view of the photoresist layer afterexposure. FIGS. 5B and 5C illustrate the cross sections of FIG. 5A.Numbers 140 and 140A represents the photoresist layer and the area notcovered by the photoresist layer respectively. FIG. 5B illustrates thecross section of FIG. 5A along the line Z–Z′. FIG. 5C illustrates thecross section of FIG. 5A along the line P–P′. The first pattern ispositioned exactly on the light-guiding channel. Due to the abovemagnification, pitch of the pattern is reduced to A′. In thisembodiment, a deep ultra violet (DUV) stepper is applied. Othersteppers, such as I-line or G-line are also applicable.

Then, etching in step S50 is performed to remove the light-guidingsubstance not covered by the photoresist layer 140. The pattern of thephotoresist layer 140 is then transferred to the light-guiding channelon the wafer. The light-guiding channel then comprises a pattern formedby the light-guiding substance 110 and the trench 105. In FIG. 6A, therelative position of the area R to the wafer 100 is illustrated. FIGS.6B and 6C illustrate the cross sections along the lines P–P′ and Z–Z′respectively in FIG. 6A. 110 represents the part filled withlight-guiding substance and 105 represent the exposed trench. It isobserved from the figures that the light-guiding channel exhibit apattern formed by light-guiding substance arranged periodically with apitch A′. A Fiber Bragg Grating optical element is then formed tofilter/select a light in wavelength A′ from the light entering thelight-guiding channel.

The following formula explains the relationship of pitch A on the maskand the pitch A′ on the wafer.A=NS=M*A′A′=NS/MWherein N is an integer, S is the highest resolution of the mask and Mis the adjustable magnification.

For example, A′ is 2005 Å when a Fiber Bragg Grating element λ_(n)=2005Å in wavelength is to be fabricated. M is preferably the magnificationclosest to the common magnification of a semiconductor process to avoidexcess adjustment and maintain stability. Therefore, M is set to be 5. Sis restricted and fixed by the electron beam of the mask-producingapparatus. In this embodiment, S is 50 Å. Under this condition, theclosest number N is 200 (or 201). Consequently, M is adjusted to 4.9875(or 5.0124). Pitch A on the mask equals 200*50=10000 Å (or201*50=100500) to obtain a pitch A′ of 2005 on the wafer. A Fiber BraggGrating optical element λ_(n)=2005 in wavelength is thus obtained.

The Magnification of the photolithography apparatus is adjusted in manyways, the easiest way is to adjust the distance between the wafer stageholding the wafer and the lens of the photolithography apparatus. Finetuning of the magnification is done by small adjustments to the heightof the wafer stage. However, it should be noted that depth of focus mustbe maintained without losing focus, or patterns may not be transferredon the photoresist layer. The mirror or mask can also be adjusted tomodify the magnifications.

In comparison to the conventional method that cannot fabricate FiberBragg Grating elements meeting various design requirements, theinvention features a method for fine tuning the magnification of aphotolithography apparatus so that Fiber Bragg Grating elements havingthe required λ_(n) are fabricated.

2^(nd) Embodiment

This embodiment is an application of the 1^(st) embodiment. A pluralityof Fiber Bragg Grating elements fabricated in the 1^(st) embodiment areformed in series, thereby enhancing accuracy and avoiding possiblevariations during the fabrication process.

FIG. 9 is a flowchart illustrating the process of the 2^(nd) embodimentof the invention. A mask and wafer are provided in step S100A and S100B.A photoresist layer is then formed on the wafer (step S120).

Next, magnification of the photolithography apparatus is adjusted to afirst magnification (step S130). First exposure is performed to thephotoresist layer 140 (step S140) without development. Then, if thenumber of Fiber Bragg Grating elements is insufficient (step S150), thewafer is moved horizontally (step S160), and magnification of thephotolithography apparatus is adjusted to a second magnification (notequal to the first magnification). A second exposure is then carried outto the photoresist layer 140. Patterns formed by two exposures are thenformed on the light-guiding channel, without overlapping one another. Inother words, multiple exposures are performed using differentmagnifications to transfer the same mask pattern to the photoresistlayer on the light-guiding channel. The number of exposures depends onthe number of Fiber Bragg Grating elements required.

Next, development is performed in step S170 to form a plurality ofsimilar patterns but have different sizes, on the photoresist layer 140.

Etching is then carried out in step S180 to remove the light-guidingsubstances not covered by the photoresist layer 140, thus transferringthe patterns on the photoresist layer onto the light-guiding channel.Every single pattern correlates to a Fiber Bragg Grating element.Therefore, a plurality of Fiber Bragg Grating elements, numbered B1˜B5,are formed in series in the light-guiding channel 110 on the wafer 100,as shown in FIG. 8A. Since different magnifications are used, FiberBragg Grating elements, having similar patterns of different sizes,exhibit different λ_(n).

The advantage associated with the Fiber Bragg Grating optical elementsformed in series is prevention of variation during the photolithographyprocess, where a single Fiber Bragg Grating element cannot accuratelyselect light of a certain wavelength.

It is assumed that variation during the process allows for 0.2%inaccuracy. Bandwidth (BW_(n)) for every single Fiber Bragg Gratingelement is only 2 Å. This means a Fiber Bragg Grating element designedto select a light 2000 Å in wavelength, the actual λ_(n) of the FiberBragg Grating element is 1996˜2004, calculated by 2000*(1±2%). If aFiber Bragg Grating element λ_(n)=1996 Å in wavelength is actuallyfabricated, FIG. 8B illustrates the relationship of Gain vs. λ_(n). Itis observed that the selected light is from 1995 (=λ_(n)−BW_(n)/2) to1997 (=λ_(n)+BW_(n)/2) in wavelength. In this case, the light of 2000 Åin wavelength is not within the range, hence cannot be accuratelyselected.

In this 2^(nd) embodiment, five Fiber Bragg Grating elements in series(B1, B2, B3, B4 and B5) having expected λ_(n) of 1996, 1998, 2000, 2002and 2004 Å are fabricated. Since the λ_(n) difference of adjacent FiberBragg Grating elements is less than the BW_(n) of a single Fiber BraggGrating element, B1˜B5 is then viewed as a equivalent Fiber BraggGrating element (Bs).

Expected λ_(sn) is 2000 Å and BW_(sn) is 10 (2*5) Å. As a result, evenif variations do occur, λ_(sn) is still in between 1996 and 2004 Å forBs. FIG. 8C illustrates the relationship of Gain vs. λ_(n) according tothe 2^(nd) embodiment of the invention. If the worst condition occurs,the actual λ_(sn) of Bs is 1996 Å., allowing Bs to select the lightbetween 1991 Å (=λ_(sn)−BW_(sn)/2) and 2001 Å (=λ_(sn)+BW_(sn)/2) inwavelength. The expected wavelength of 2000 Å is thus successfullyfiltered by Bs. In other words, even if the worst condition occurs, theequivalent Fiber Bragg Grating element fabricated by the method providedin this invention still selects light of the required wavelength (2000Å). In FIG. 8C, a wavelength of 2000 Å still falls in the range ofselected wavelength. Consequently, variations during the process areovercome.

In other words, an equivalent Fiber Bragg Grating element, formed by aplurality of Fiber Bragg Grating elements in series, effectivelyincreases the bandwidth so that variation during the process iseliminated. Design requirements are thus satisfied.

The equivalent Fiber Bragg Grating element should be designed carefullyto avoid filtering out the light that should not have been selected. Forexample, if the selected light exhibits a wavelength of 2000 Å, and thelight not to be filtered is 1990 Å. It is assumed that variation ofλ_(n) during process is Δ λ. Therefore, considering the worst situationwhere λ_(sn) is 2000−(Δλ), the shortest wavelength filtered is (2000−(Δλ)−BW_(sn)/2) Å. This value must be greater than 1990. By doing so,filtering light of 1990 Å can be avoided.

The equivalent Fiber Bragg Grating element (Bs) fabricated in the2^(nd)embodiment is an optical notch filter correlated to a certainbandwidth (BW_(n)), a notch frequency (f_(n)) and a corresponding notchwavelength (λ_(n)). As shown in FIG. 8A, the Fiber Bragg Gratingelements B1˜B5 formed in series are in the light-guiding channel 110 onthe wafer 100. Because of different magnifications for each exposure,every single Fiber Bragg Grating element is different from the others insize but have similar patterns. As a result, different wavelengths λ_(n)for every single Fiber Bragg Grating element is fabricated. Byfabricating a series of Fiber Bragg Grating elements to form anequivalent Fiber Bragg Grating element, one equivalent notch wavelengthand one equivalent bandwidth are obtained. By doing so, the equivalentbandwidth of an equivalent Fiber Bragg Grating element is greater thanthat of any Fiber Bragg Grating element.

According to the method provided in this invention, patterns on a maskcan be transferred to wafers by any magnification, either integers ornon-integers, by fine tuning of the photolithography apparatus.Consequently, a wide variety of design requirements can be satisfied.Fiber Bragg Grating elements fabricated thereof exhibit high accuracy.In addition, variation during process is overcome by fabricating anumber of Fiber Bragg Grating elements in series so that bandwidth iseffectively enlarged to ensure filtering out of the required wavelength.

While the invention has been described by way of example and in terms ofthe preferred embodiments, it is to be understood that the invention isnot limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications and similar arrangements (aswould be apparent to those skilled in the art). Therefore, the scope ofthe appended claims should be accorded the broadest interpretation so asto encompass all such modifications and similar arrangements.

1. A method for fabricating a Fiber Bragg Grating element, comprising: (a) providing a mask having a predetermined pattern and a wafer, wherein a light-guiding channel filled with light-guiding substance is formed on the wafer, and a photoresist layer is formed on the wafer; (b) adjusting a magnification of a photolithography apparatus to a first magnification and transferring the predetermined pattern on the mask to the photoresist layer on the wafer to form a first pattern; and (c) removing the light-guiding substance not covered by the photoresist layer so that the first pattern is transferred to the light-guiding channel thus forming the Fiber Bragg Grating element, which picks out a light of a specific wavelength.
 2. The method as claimed in claim 1, wherein the mask comprises a glass substrate.
 3. The method as claimed in claim 1, wherein the predetermined pattern is made of Cr.
 4. The method as claimed in claim 1, further comprising: (d) adjusting the magnification of the photolithography apparatus to a second magnification so that the predetermined pattern is transferred to the photoresist layer to form a second pattern, wherein the second magnification is not equal to the first magnification, and the first pattern and the second pattern are formed on the light-guiding channel without overlapping one another; wherein the first pattern and the second pattern are simultaneously transferred in step (c) to the light-guiding channel on the wafer.
 5. The method as claimed in claim 1, wherein the first magnification is a positive integer or a non-positive integer. 